Because of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. To meet the recent concern about the environmental problem, the range of utilization of magnets has spread to cover large-size equipment such as industrial equipment, electric automobiles and wind power generators. This requires further improvements in performance and electric resistance of Nd—Fe—B magnets.
Eddy current is one of factors that reduce the efficiency of motors. Although eddy current mainly generates in a magnetic core, the eddy current of the magnet itself becomes more noticeable as the motor becomes larger in size. Especially in the case of an interior permanent magnet (IPM) motor having a rotor wherein slots are perforated in a laminate of magnetic core plies stacked with interleaving insulating films and permanent magnets are in sliding fit with the slots, the magnets facilitate conduction between core plies, allowing a greater eddy current to generate. There have been proposed several methods for coating magnets with insulating resins. There are left some problems that resin coatings can be rubbed and stripped off when magnets are slidingly inserted into slots, and the “shrinkage fit” technique of securing magnets by utilizing thermal expansion is not applicable.
Also there have been proposed several methods of processing magnets into thin plates like the core plies, and stacking magnet plates with interleaving insulating plates. These methods are not widespread because of low productivity and increased costs.
Therefore, it is rather effective to increase the electric resistance of permanent magnets themselves, and a number of methods have been proposed. Since Nd—Fe—B permanent magnets are metallic materials, they have a low electric resistance, as demonstrated by a resistivity of 1.6×10−6 Ω−m. In a typical prior art approach, a number of particles of high electric resistance substance such as rare earth oxide are dispersed in a magnet to induce more electron scattering by which the resistance of the magnet is increased. On the other hand, this approach reduces the volume fraction in the magnet of the primary phase of Nd2Fe14B compound contributing to magnetism. There is a contradictory problem that the higher the resistance, the more outstanding become the magnetic property losses.
Japanese Patent No. 3,471,876 discloses a rare earth magnet having improved corrosion resistance, comprising at least one rare earth element R, which is obtained by effecting fluorinating treatment in a fluoride gas atmosphere or an atmosphere containing a fluoride gas, to form an RF3 compound or an ROxFy compound (wherein x and y have values satisfying 0<x<1.5 and 2x+y=3) or a mixture thereof with R in the constituent phase in a surface layer of the magnet, and further effecting heat treatment at a temperature of 200 to 1,200° C.
JP-A 2003-282312 discloses an R—Fe—(B,C) sintered magnet (wherein R is a rare earth element, at least 50% of R being Nd and/or Pr) having improved magnetizability which is obtained by mixing an alloy powder for R—Fe—(B,C) sintered magnet with a rare earth fluoride powder so that the powder mixture contains 3 to 20% by weight of the rare earth fluoride (the rare earth being preferably Dy and/or Tb), subjecting the powder mixture to orientation in a magnetic field, compaction and sintering, whereby a primary phase is composed mainly of Nd2Fe14B grains, and a particulate grain boundary phase is formed at grain boundaries of the primary phase or grain boundary triple points, said grain boundary phase containing the rare earth fluoride, the rare earth fluoride being contained in an amount of 3 to 20% by weight of the overall sintered magnet. Specifically, an R—Fe—(B,C) sintered magnet (wherein R is a rare earth element, at least 50% of R being Nd and/or Pr) is provided wherein the magnet comprises a primary phase composed mainly of Nd2Fe14B grains and a grain boundary phase containing a rare earth fluoride, the primary phase contains Dy and/or Tb, and the primary phase includes a region where the concentration of Dy and/or Tb is lower than the average concentration of Dy and/or Tb in the overall primary phase.
These proposals, however, are still insufficient in improving surface electric resistance.
JP-A 2005-11973 discloses a rare earth-iron-boron base magnet which is obtained by holding a magnet in a vacuum tank, depositing an element M or an alloy containing an element M (M stands for one or more rare earth elements selected from Pr, Dy, Tb, and Ho) which has been vaporized or atomized by physical means on the entirety or part of the magnet surface in the vacuum tank, and effecting pack cementation so that the element M is diffused and penetrated from the surface into the interior of the magnet to at least a depth corresponding to the radius of crystal grains exposed at the outermost surface of the magnet, to form a grain boundary layer having element M enriched. The concentration of element M in the grain boundary layer is higher at a position nearer to the magnet surface. As a result, the magnet has the grain boundary layer in which element M is enriched by diffusion of element M from the magnet surface. A coercive force Hcj and the content of element M in the overall magnet have the relationship:Hcj≧1+0.2×Mwherein Hcj is a coercive force in unit MA/m and M is the content (wt %) of element M in the overall magnet and 0.05≦M≦10. This method, however, is extremely unproductive and impractical.